Various types of biometric systems are used more and more in order to provide for increased security for accessing an electronic device, thereby providing an enhanced user convenience. In particular fingerprint sensors have been successfully integrated in such devices, for example, thanks to their small form factor, high performance and user acceptance. Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
All capacitive fingerprint sensors provide a measure indicative of the capacitance between several sensing elements and a finger placed on the surface of the fingerprint sensor. Acquisition of a fingerprint image is typically performed using a fingerprint sensor comprising a plurality of sensing elements arranged in a two-dimensional manner, and a block based technique may be applied to the fingerprint sensor for acquiring a fingerprint image, where the blocks of sensing elements are sampled sequentially. As an example, a block of eight sensing elements adjacently arranged in one row and connected to a plurality of A/D conversion means, for example including include at least one amplifier having a controllable gain, may be sampled at the same time.
Generally there is a large variation in the intensity in the acquired fingerprint image, for example due to different finger types, finger conditions, different users, different sensing conditions, or based on manufacturing process variations. Thus, it is typically necessary to adjust the dynamic range of the A/D conversion means and/or to control the gain of the amplifier. Accordingly, an iterative/repeated sampling process is performed using different the settings for the A/D conversion means and/or the amplifier, with the purpose of acquiring a “good quality” fingerprint image. The problem with such a process is that it will be time consuming and thus inconvenient for the user.
An exemplary implementation for trying to overcome this problem is disclosed in U.S. Pat. No. 6,259,804. U.S. Pat. No. 6,259,804 disclose the use of a gain processor that may comprise histogram generating means for generating a histogram of previously performed A/D conversions. Based on the generated histogram it may be possible to control the range of the A/D conversion means, to set the gain and for controlling the offset of the amplifier.
Even though U.S. Pat. No. 6,259,804 introduces an interesting approach to faster acquisition of a good quality fingerprint image, the disclosed approach will not make any distinction between previously successfully acquired fingerprint images, thereby making the approach highly ineffective in the general user environment. Thus, there appears to be room for further improvement in regards to the determination of control parameters for fast acquisition of a good quality fingerprint image.